1. Field of the Invention
The present invention relates to surface acoustic wave filters used as high-frequency devices.
2. Description of the Related Art
With the recent spread of automobile telephones and portable telephones, the necessity for small-sized, high-performance high-frequency filters is increased. As the high-frequency filters, a dielectric filter and a surface acoustic wave filter are conventionally known. The latter surface acoustic wave filter is suitable for small size and high performance.
Examples of the surface acoustic wave filter include surface acoustic wave filters of an interdigitated interdigital transducer type, a multiple-mode type, and a composite-type (a ladder connection type). Each of the surface acoustic wave filters can realize a band-pass filter at a frequency of not less than 800 MHz. The composite-type surface acoustic wave filter has been paid attention to in that it has a low loss and it requires no matching circuit.
In the composite type surface acoustic wave filter, two one-port resonators 22 each formed by an interdigital transducer 22a and grating reflectors 22b on a piezoelectric substrate (e.g., LiNbO.sub.3) 21 constitute one filter functional unit 23, as shown in FIG. 13. One of the one-port resonators 22 is electrically connected in series with a signal line 24 (this is referred to as a series arm resonator 22S), and the other one-port resonator 22 is electrically connected in parallel with the signal line 24 (this is referred to as a parallel arm resonator 22P). FIG. 14 illustrates the basic construction of the filter functional unit 23.
The composite type surface acoustic wave filter realizes a band-pass filter utilizing the difference in impedance between the series arm resonator 22S and the parallel arm resonator 22P. The principle will be briefly described with reference to FIGS. 15A and 15B. Let jXs be the impedance of the series arm resonator 22S, jXp be the impedance of the parallel arm resonator 22P, f.sub.ap be the antiresonance frequency of the parallel arm resonator 22P and f.sub.rp be the resonance frequency thereof, and f.sub.as be the antiresonance frequency of the series arm resonator 22S and f.sub.rs be the resonance frequency thereof. In this case, the antiresonance frequency f.sub.ap and the resonance frequency f.sub.rs are made approximately equal to each other, as shown in FIG. 15A, whereby filter characteristics taking the antiresonance frequency f.sub.as and the resonance frequency f.sub.rp as poles centered around the approximately equal frequencies are obtained, as shown in FIG. 15B.
The number of resonators 22 is set on the basis of the insertion loss, the attenuation outside the band, and the like of the filter. If 36.degree.-rotated Y-cut X-propagating LiTaO.sub.3 is used as a piezoelectric substrate 21 in such a construction that filter functional units 23 in three stages are provided using three series-arm resonators 22S and three parallel-arm resonators 22P, a low-loss filter having a minimum insertion loss of not more than 2 dB and having attenuation outside the band of not less than 25 dB in a frequency band of 800 MHz is obtained. In this construction, the impedance of the series-arm resonator 22S is zero and the impedance of the parallel-arm resonator 22P is sufficiently more than 50 .OMEGA. in the passband, whereby impedance matching is achieved, and no matching circuit is required.
In the composite-type surface acoustic wave filter, the degree of freedom for changing the band width is low from its filter principle. Specifically, it is important to make the resonance frequency f.sub.rs of the series arm resonator 22S and the antiresonance frequency f.sub.ap of the parallel arm resonator 22P approximately equal to each other. If an attempt is made to obtain a filter having a large band-width without making the resonance frequency f.sub.rs and the antiresonance frequency f.sub.ap approximately equal to each other, the difference in the resonance frequencies .DELTA.f (f.sub.rs -f.sub.rp) must be increased, as shown in FIGS. 16A and 16B. In this case, the deviation between the antiresonance frequency f.sub.ap and the resonance frequency f.sub.rs causes a ripple in the band. When the difference in the resonance frequencies .DELTA.f is decreased, as shown in FIGS. 17A and 17B, the passband is brought into a convex shape.
In order to change the band-width while obtaining proper filtering characteristics, therefore, the difference between the resonance frequency and the antiresonance frequency of each of the resonators 22 must be changed. one method of changing the difference in frequencies includes a method of changing the electromechanical coupling factor of the piezoelectric substrate 21. This method utilizes the fact that the difference between the resonance frequency and the antiresonance frequency of the resonator 22 is increased if the electromechanical coupling factor is increased, while being decreased if the electromechanical coupling factor is decreased. Therefore, a substrate having a large electromechanical coupling factor may be used when a filter having a large band-width is required, while a substrate having a small electromechanical coupling factor may be used when a filter having a small band-width is required.
In the conventional method of changing the difference in frequencies by changing a substrate, however, the following problem occurs. That is, as a piezoelectric substrate used for a high-frequency filter having a frequency of not less than 800 MHz, only two types of substrates, i.e., 36.degree.-rotated Y-cut X-propagating LiTaO.sub.3 (electromechanical coupling factor k.sup.2 =0.047) and 64.degree.-rotated Y-cut X-propagating LiNbO.sub.3 (electromechanical coupling factor k.sup.2 =0.11) are useful. Therefore, it is difficult to realize a filter sufficiently coping with a frequency band-width (e.g., 33 MHz, 25 MHz, 17 MHz) required in each communication method.
An object of the present invention is to provide a surface acoustic wave filter whose degree of freedom in design is high in terms of band-width.
As an interdigitated interdigital transducer type filter in which a plurality of input and output electrodes are arranged on the same propagation path, techniques disclosed in Transactions of The Institute of Electrical Engineers of Japan, 111-C, 9, pp.396-403 and Japanese Patent Laid-Open Gazette No. 270309/1991 are known. As a modecoupling type filter utilizing modecoupling of a surface acoustic wave resonator, techniques disclosed in Journal of Institute of Communication Engineering of Japan, A Vol. j76A No. 2, pp.227-235 93/2 are known. The modecoupling type filter has frequency-attenuation characteristics as shown in FIG. 18. When the surface acoustic wave filter is used as a band-pass filter, examples of required characteristics include a passband width, an insertion loss, and attenuation outside the band. Further, in a high-frequency filter, it is desired to reduce the insertion loss to the utmost.
Another object of the present invention is to provide an interdigitated interdigital transducer type surface acoustic wave filter having a reduced insertion loss.